The present invention generally relates to optical recording and reproducing, and more particularly, to an optical recording and reproducing method which employs a converting element means for converting a wavelength of a light beam produced from a light source such as a semiconductor laser or the like into a shorter wavelength, and an apparatus employed for effecting said method.
Conventionally, there has been available an optical recording and reproducing apparatus arranged to project a light beam produced from a light source such as a semiconductor laser, etc. and properly converged, onto a disc-like recording medium rotating at a predetermined rotational speed, so as to thereby record a signal onto the recording medium or to reproduce the signal recorded on the recording medium.
On such a recording medium, very fine tracks, e.g. 0.6 .mu.m in width, and 1.6 .mu.m in pitch are formed in a spiral shape or concentric configuration. For recording signals on the recording medium, the intensity of the light beam is modulated to be either high or low, while, for reproducing signals recorded on the recording medium, light reflected from or transmitted through the recording medium is converted into a photoelectric current by a photo-detector.
In the field of the optical recording and reproducing apparatus, it has been a requirement of times to record a large capacity of information on a limited surface of the recording medium. In order to meet such a requirement, it may be so arranged to reduce a spot diameter of the light beam on the recording medium, so as to thereby increase the linear density in the directions of the track pitch and tracks. The light beam spot diameter D on the recording medium is determined by an equation: EQU D=k.sub..theta. .times..lambda./NA
where k.sub..theta. is a constant, .lambda. is a wavelength of the light beam, and NA is a numerical aperture of a lens converging the light beam onto the recording medium. Accordingly, recording and reproducing of signals at high density become possible, if the wavelength of the light beam is shortened.
As one practice for obtaining a light beam of a short wavelength, there has been known a method in which a solid state laser crystal such as YAG (yttrium-aluminum-garnet) or the like is excited by a light beam such as the semiconductor laser etc., and the light beam generated by the solid state laser crystal is converted into a light beam of a half wavelength through employment of a second harmonic generator (SHG) element (referred to as an SHG element hereinafter). For example, a light beam with a wavelength of 1064 nm is generated by a YAG laser referred to above, and is converted into a light beam of 532 nm by the second harmonic generator (SHG) element (as disclosed, for example, in "Nikkei new materials" magazine, Aug. 20, 1990 issue, pages 32 to 33, and 60 to 68). However, such a practice for obtaining the light beam of a short wavelength has a disadvantage in that it is low in converting efficiency, and for example, even when a semiconductor laser of 809 nm at 100 mW is employed, the light beam of 532 nm is obtained at only about 2 to 3 mW.
On the other hand, in an optical recording and reproducing apparatus, a light intensity of 1 mW or thereabout on the recording medium is sufficient to reproduce the recorded information, but for recording information, a light intensity on the order of from 10 mW to 20 mW on the recording medium is required, and it has been difficult to construct an efficient recording and reproducing apparatus by the known arrangement.